1. Field of the Invention
The present invention relates to an output circuit which outputs a voltage that corresponds to the ON state (OFF state) of a power transistor.
2. Description of the Related Art
In the field of electronic circuits, half-bridge circuits, H-bridge circuits, and source follower circuits are widely employed. FIG. 1 is a circuit diagram which shows the configuration of an ordinary half-bridge circuit 400. The half-bridge circuit 400 outputs, via an output terminal POUT, the power supply voltage Vdd or the ground voltage 0V, or an intermediate voltage between the two.
The half-bridge circuit 400 includes a high-side transistor M1H and a low-side transistor M1L, which each function as a power transistor, and pre-drivers 402 and 404 configured to drive these power transistors. The high-side transistor M1H and the low-side transistor MIL are connected in series between the power supply terminal PVCC and the ground line PGND. The high-side transistor M1H is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor), and the low-side transistor M1L, is a P-channel MOSFET.
The pre-drivers 402 and 404 switch on the high-side transistor M1H and the low-side transistor M1L, in a complementary manner according to a control signal S1. When the high-side transistor M1H is switched to the ON state, the output voltage Vout is switched to the power supply voltage Vdd, and when the low-side transistor M1L is switched to the ON state, the output voltage Vout is switched to the ground voltage 0 [V].
The pre-driver 402 for the high-side transistor supplies, to a control terminal (gate) of the high-side transistor M1H, a gate voltage Vg1 at the high level (Vdd) or the low level (VL) according to the control signal S1. Similarly, the pre-driver 404 for the low-side transistor supplies, to the gate of the low-side transistor M1L, a gate voltage Vg2 at the high level (VH) or the low level (0 V) according to the control signal S1.
There is a need to determine the electric potential VL of the low level of the gate voltage Vg1 giving consideration to the gate-source breakdown voltage of the high-side transistor M1H. Specifically, the voltage VL of the gate voltage Vg1 is designed such that the gate-source voltage (Vdd−VL) does not exceed the gate-source breakdown voltage.